Heat is far worse than energy or physical from my experience.
Especially if you compare weapons like for exampe Corrupt Light (heat) with the energy equivalent Malice Beam.
Corrupt Light only deals 93 heat damage as maxed-out myth while Malice Beam deals 117 or so energy damage as maxed-out myth.
At the same time the modules for increasing heat / energy regeneration have the same values so recovering energy is harder than heat because energy weapons reduce energy faster than heat weapons increase heat.
And so as heat user you either need high energy regen and max energy to keep your weapons functioning but then are weak to physical mechs with high damage because of your own low max. HP
or you equip some heavy plates to increase max. HP but suck on energy regen and therefore get owned by any energy focused mech.
So from my point of view I would say that energy and physicals must be nerfed or heat based weapons must be strengthened.
But I do not call for any nerf or buff because I simply decided to first take my time to build a heat focused mech and then switch over to either an energy or a physical focused mech afterwards.
Then the advantages or disadvantages will cancel each other out as I will then be able to switch between mechs.